Magnetic resonance rheology is an imaging method in which magnet resonance tomography is used to characterize the mechanical properties of tissue. For this purpose, tissue is driven to oscillate mechanically during imaging, leading to certain effects which cause an imaging contrast. Low-frequency mechanical waves are coupled into the tissue and visualized via a magnetic resonance sequence which is phase-locked to the mechanical excitation. Palpation has turned into the assessment of an objective absolute physical quantity, whose diagnostic value can be quantified.
This information can be used to distinguish tissue, i.e., healthy, malign, and etc., based on its viscoelastic properties and leads to a substantial rise in specificity, e.g., for cancer diagnosis. A number of different transducers for employing the mechanical oscillation to the tissue have been proposed and demonstrated namely electromagnetic designs, which make use of the B0-field inside the MR scanner. Piezo-driven transducers or pneumatic designs were proposed for clinical application. The US-patent application US2011/025333 discloses a pneumatic driver located remotely from the MRI scanner. The driver produces shear waves in the subject for performing MR elastography.
The oscillation of the tissue is achieved by attaching a mechanical oscillator to the patient close to the imaging region of interest. The oscillator can be based on an electromechanical converter, like an AC current driven coil, oscillating in a static magnetic B0 field. However, it could be based on any other principle which generates mechanical oscillations of tissue in the required manner in a controlled way. (hydrodynamic, piezoelectric, pneumatic, . . . actuators).
By adjusting a proper input signal the oscillator generates the mechanical waves in the body. Today, the magnitude of the oscillation is manually adjusted based on experiences, e.g. by setting the electrical current through the coil of the electromechanical oscillator mentioned. Changes in resistivity of feeding and coil conductor provoke drifts of currents and thus the amplitude of the oscillator. Different heating of the resistive primary and compensation coil leads to misalignment of B0 compensation, which leads to B0 drifts and B0 artifacts.